1. Field of the Invention
The present invention relates to a process for producing N-(1-alkoxyethyl)carboxylic amides. It specifically relates to a process for producing N-(1-alkoxyethyl)carboxylic amides which are useful for analytical purposes, as intermediates for various agricultural chemicals and medicines, and which are also production intermediates for N-vinylcarboxylic amides which are very useful as monomers for N-vinylcarboxylic amide-based polymers that can be employed in water absorbers and thickeners, and as synthetic materials for chemical agents such as taurine and cysteamine.
The present invention further relates to a process for producing N-vinylcarboxylic amides from N-(1-alkoxyethyl)carboxylic amides, and to a process for producing N-vinylcarboxylic amide-based polymers using the obtained N-vinylcarboxylic amides as monomers.
2. Description of the Related Art
A number of conventional processes have been proposed for synthesizing N-(1-alkoxyethyl)carboxylic amides. Considering the starting materials used in such processes, they may be largely classified into processes using acetals, processes using ethylidene biscarboxylic amides and processes using acetaldehyde.
A process with dimethylacetal as the starting material is disclosed in U.S. Pat. No. 4,554,377 as a process by which dimethylacetal and a carboxylic amide are reacted in the presence of a strong acid such as methanesulfonic acid or sulfuric acid, or a strongly acidic ion-exchange resin. This process, however, has a drawback in that the dimethylacetal must be synthesized separately and isolated for use, while high yields of N-(1-alkoxyethyl)carboxylic amides can only be obtained by diluting the starting composition to a very high degree of 20 moles of dimethylacetal to 1 mole of the carboxylic amide, thus notably lowering productivity. Also, Japanese Unexamined Patent Publication No. 2-009851 describes a process whereby N-(1-alkoxyethyl)formamides are synthesized in the same manner from a formamide and acetal, but it entails the same problems.
A process using an ethylidene biscarboxylic amide as the starting material is described in Japanese Unexamined Patent Publication Nos. 1-100153 and 2-304053. This process gives the desired N-(1-alkoxyethyl)carboxylic amide both easily and at a high yield by reacting an alkanol with an ethylidene biscarboxylic amide which can be easily produced from readily obtainable and low-cost acetaldehyde and carboxylic amides or from vinyl ether and carboxylic amides. However, among problems associated with this process is the need for a complicated procedure including extraction, in order to isolate the N-(1-alkoxyethyl)carboxylic amide after completion of the reaction, as well as the need for the two steps of synthesis of the ethylidene biscarboxylic amide and synthesis of the N-(1-alkoxyethyl)carboxylic amide.
A process using acetaldehyde as the starting material is described in Japanese Examined Patent Publication No. 6-17351 and Japanese Unexamined Patent Publication No. 63-96160. Despite a relatively high yield for the desired N-(1-alkoxyethyl)carboxylic amide, acetal and ethylidene bisacetamide by-products result. Japanese Examined Patent Publication No. 5-81581 describes a similar process. Here there is discussed the possibility recycling the acetal by-product for reuse in the reaction system during the production process for N-(1-alkoxyethyl)carboxylic amide, but no concrete disclosure is given regarding the reaction results. Also, the reaction results are not of a sufficient level.
On the other hand, Japanese Unexamined Patent Publication No. 6-100515 describes a process for producing a desired N-(1-alkoxyethyl)carboxylic amide at a relatively high yield in the presence of an acetal and ethylidene biscarboxylicamide.
Since the process using acetaldehyde in this manner allows synthesis of the desired N-(1-alkoxyethyl)carboxylic amide at a relatively high yield by single-step reaction from the 3 easily obtainable and inexpensive compounds carboxylic amide, acetaldehyde and alcohol in the presence of a strongly acidic catalyst, it is expected to be an industrially advantageous process for producing N-(1-alkoxyethyl)carboxylic amide.
Japanese Unexamined Patent Publication Nos. 62-289549 and 63-96160 disclose a process in which a desired N-(1-alkoxyethyl)acetamide is obtained at a high yield, by using an acetamide, acetaldehyde and an alcohol as the starting material, in the presence of an acidic catalyst. This process provides an advantage in that the reaction products and catalyst can easily be separated, since a strongly acidic ion-exchange resin of a heterogeneous catalyst is used.
However, when a strongly acidic ion-exchange resin is used, it may be inactivated during the use. This is supposed to be due to the presence of impurities such as a trace amount of basic ions by-produced during the reaction, metal ions or basic substances contained in the starting material, and the like. Therefore, this process is industrially unsatisfactory in the view point of the discontinuity of production due to the inactivation of the catalyst, the cost for regenerating the inactivated catalyst and the like.
Further, the above-mentioned Japanese Unexamined Patent Publication No. 6-100515 discloses the use of a homogeneous catalyst. However, the process includes a problem of disposal of the neutralized salt, since it is necessary to use a large amount of an acidic catalyst.
Incidentally, since all of these processes are equilibrium reactions, none can completely convert all of the carboxylic amide to N-(1-alkoxyethyl)carboxylic amide. Consequently, the carboxylic amide starting material is present in the reaction solution in addition to the N-(1-alkoxyethyl)carboxylic amide. Generally speaking, the N-(1-alkoxyethyl)carboxylic amide and its carboxylic amide starting material have very similar physical properties including vapor pressure, and their purification has been particularly difficult when the alkoxy groups are lower aliphatic alkoxy groups such as methoxy, ethoxy and isopropoxy.
When necessary, obtained N-(1-alkoxyethyl)carboxylic amides are subjected to thermal cracking or catalytic cracking reaction to accomplish alcohol elimination reactions, for production of N-vinylcarboxylic amides. This reaction also is an equilibrium reaction, and thus the N-(1-alkoxyethyl)carboxylic amide cannot be completely converted to N-vinylcarboxylic amide. Consequently, the resulting reaction solution contains the unreacted N-(1-alkoxyethyl)carboxylic amide and carboxylic amide as major impurities in addition to the N-vinylcarboxylic amide.
Various processes for recovery and purification of the N-vinylcarboxylic amide from the reaction solution have been proposed. For example, when the N-vinylcarboxylic amide is obtained in a crystallizing procedure, not all of the N-vinylcarboxylic amide can be recovered in principle, and the resulting mother liquor is composed of the N-(1-alkoxyethyl)carboxylic amide and carboxylic amide, which includes the N-vinylcarboxylic amide.
On the other hand, when the N-vinylcarboxylic amide is obtained by a separating procedure such as distillation or extraction, it is quite difficult to separate it because its physical properties are similar to those of the N-(1-alkoxyethyl)carboxylic amide and carboxylic amide. Consequently, in most cases there are obtained the recovered and purified N-vinylcarboxylic amide and a distilled fraction or extraction residue containing the N-(1-alkoxyethyl)carboxylic amide and carboxylic amide also containing the N-vinylcarboxylic amide.
Thus, recovery of such mixtures composed of N-(1-alkoxyethyl)carboxylic amides and carboxylic amides containing N-vinylcarboxylic amides as useful substances in an efficient and easy manner has been a problem.
As mentioned above, in conventional processes for synthesizing N-(1-alkoxyethyl)carboxylic amides, many improvements are necessary in terms of the yield, formation of by-products, difficulty in obtaining starting materials, catalyst life and complexity of the reaction and purification steps, and it has been difficult to obtain the N-(1-alkoxyethyl)carboxylic amides at high concentration with a low content of unreacted carboxylic amide.
In addition, even when an N-(1-alkoxyethyl)carboxylic amide is used for the production of an N-vinylcarboxylic amide, the method has not been satisfactory in terms of the yield, by-product control and complexity of the entire production process including the subsequent steps of recovery and purification.